Compressors used in many refrigeration systems generally require close control over the compressor motor speed in order to maintain the system within desired limits under varying load conditions. Such refrigeration systems may include compressors where motor speed is varied via incorporation with variable frequency drives (VFD) that contain power electronic components that produce heat and require cooling.
With VFD compressors utilizing rolling element bearings, the oil viscosity required to ensure reliable operation is a function of the rotating speed. Some compressors operate at low speeds and may require oil with a high viscosity. To achieve high viscosity with typical refrigerant and oil mixtures, obtaining an optimal temperature for the given pressure of the mixture is needed. This has typically been achieved with oil cooling or supplemental electric heat. Accordingly, it is desirable to achieve a required oil viscosity without the need for supplemental electric heat or excessive parasitic losses from oil cooling to achieve more efficient operation.